Creep-Induced Weld Cracking in 12Cr1MoVG Steel Superheater Tee

Literature Overview

This paper by Yao Yufeng, Yang Junsheng, and Ding Ao, published in Physical Testing and Chemical Analysis (2026, Vol. 62, No. 6), reports on the investigation of multiple transverse cracks found in the weld of a 12Cr1MoVG steel superheater tee during a power plant overhaul. The authors employed macroscopic observation, chemical composition analysis, hardness testing, metallographic examination, and scanning electron microscopy (SEM) to determine the root cause. The study is highly relevant to engineers working on high-temperature power plant piping and components, where creep damage is a dominant failure mechanism over long-term service.

Creep Damage Mechanism

The investigation revealed that the crack region and its immediate vicinity contained a high density of creep voids and creep cracks. Away from the crack location, only a small number of creep voids and microcracks were observed. The microstructure at the rupture surface and crack locations showed severe aging, indicating prolonged exposure to elevated temperatures beyond the material's optimal service range. The authors concluded that the weld experienced creep damage under the combined action of internal pressure hoop stress and welding residual stress. The creep voids nucleated at grain boundaries and second-phase particles, grew under sustained stress, and eventually coalesced to form through-thickness cracks.

The material 12Cr1MoVG is a low-alloy steel specifically designed for high-temperature service in power plant applications, typically used in superheater and reheater tubes and components operating at temperatures up to 580 degrees Celsius. The creep resistance of this material depends on its microstructure, particularly the stability of the precipitate phases and the grain boundary condition. When welding residual stresses are superimposed on the operating stresses, the effective stress at the weld exceeds the creep threshold, accelerating void nucleation and growth.

Welding Residual Stress and Creep Interaction

The interaction between welding residual stress and creep is a well-documented phenomenon in high-temperature pressure equipment, but its severity in tee welds deserves special attention. The following table summarizes the key factors:

The transverse orientation of the cracks is particularly significant. Transverse cracks in a tee weld are oriented perpendicular to the longitudinal axis of the pipe, which means they are perpendicular to the hoop stress direction. This orientation is consistent with the mechanism of creep crack growth, where cracks propagate in the plane of maximum tensile stress. The presence of welding residual stress in the hoop direction further exacerbates this condition.

Engineering Practice and Recommendations

For power plant engineers, this case underscores several critical practices. First, post-weld heat treatment (PWHT) is essential for 12Cr1MoVG welds to relieve welding residual stresses. According to ASME Section VIII Div. 1 and ASME B31.1, PWHT for 12Cr1MoVG typically involves holding at 700 to 750 degrees Celsius for a duration proportional to the wall thickness, followed by controlled cooling. Failure to perform adequate PWHT leaves residual stresses that can be as high as 300 to 400 MPa, which significantly reduces creep life.

Second, in-service monitoring of superheater tees should include periodic ultrasonic testing (UT) for creep damage. According to API 579-1/ASME FFS-1, the evaluation of creep damage in power plant components requires consideration of creep voids, creep cavitation, and creep crack growth. The presence of creep voids detected by UT should trigger a detailed engineering assessment, including metallurgical examination of a coupon if feasible.

Third, the operating temperature and pressure should be reviewed against the material's creep strength. For 12Cr1MoVG, the allowable stress at 550 degrees Celsius is approximately 90 MPa according to ASME Section II Part D. If the operating pressure combined with residual stress exceeds this value, accelerated creep damage is expected. Engineers should consider the possibility of material upgrade to 9Cr-1Mo or P91 steel for critical tee locations where creep damage is a concern.

This case is a textbook example of how welding residual stress can dramatically reduce the creep life of a high-temperature component. The lesson for engineers is that weld quality in creep-critical applications is not merely about the absence of welding defects; it is about the complete elimination of residual stress through proper PWHT and the selection of materials with adequate creep resistance for the specific service conditions.